try autolite 3924, regular reach, old-school style spark plugs.
the long-reach plugs recommended in most books are TOO hot of plug (rough idle misfire) this includes autolite 985 and champion R12YC plugs...those are all long reach and if you have a rough idle when motor is warmed up...caused by too hot heat range long-reach plugs.
go to store and look at all these plugs...you'll see what i mean.
Newly Painted! Emerald Green '96 XJ 4x4 4.0 HO 5-spd
3" Lift + 31" Tires / D35 TracLok / NP231
Drilled out Stock Airbox / Gauge Cluster Upgrade
Magnaflow Muffler / High-flo Cat / MSD Coil